First Use of GPS Satellites for Beam Calibration of Radio Dish Telescopes

Abstract

We present results from the first application of the Global Navigation Satellite System (GNSS; e.g., the Global Positioning System, GPS) for radio beam calibration using a commercial GNSS receiver with the Deep Dish Development Array (D3A) at the Dominion Radio Astrophysical Observatory (DRAO). Several GNSS satellites pass through the main and sidelobes of the beam each day, enabling efficient mapping of the 2D beam structure. Due to the high SNR and abundance of GNSS satellites, we find evidence that GNSS can probe several sidelobes of the beam through repeatable measurements of the beam over several days. Over three days of measurements, the smallest observed difference in the primary beam's main lobe was 0.56 dB-Hz. We also compare our results in the sidelobes to simulations and find rough agreement in shape. When scaling the observations and simulations to match in the main lobe power levels, we find deviations in at least one of the first few nulls of approximately 5 dB or less. There is saturation in the main lobe for most satellites, which can likely be mitigated by better attenuation before the receiver input. We compare our work to other satellite systems that have been successful and are likely complementary to this technique. However, GNSS offers key advantages, including continuous transmission, broader frequency coverage relevant to CHORD, SKA-mid, and the DSA-2000, as well as more frequent satellite passes, making it a promising calibration method. These results also motivate further development of this technique for radio astronomy applications.

Figures (12)

• Figure 1: Model of a single feed and dish as simulated in CST Microwave Studio, (a) zooming in on the feed, and (b) showing the whole model. The dish was simplified to an analytic parabola and all support structures were omitted, to enable a more efficient simulation.
• Figure 2: Peak-normalised beams in polar coordinates for the E and H planes at (a) 1.1764 GHz and (b) 1.5754 GHz, as simulated with CST Microwave Studio using the models shown in Figure \ref{['fig:cst_models']}. Those frequencies correspond to GPS L5 and L1, respectively.
• Figure 3: Observation setup at the DRAO; (a) one of the three 6 m D3A dishes---CHORD will include 512 similar dishes situated next to the main CHIME site---(b) backend of the D3A. The bottom crate slot contains the triplexer used in data collection. The middle part of the crate shows the ICE boards normally used in processing the D3A signal.
• Figure 4: Plot of GPS SVN 61's directivity at emission, i.e., characterised from the on-satellite antenna panel. The zoomed in plot exemplifies the fact that there is only up to about 2 dB power loss across the Earth. The data used to make this plot were taken in 10 degree increments from the US Coast Guard Navigation Website.
• Figure 5: The Septentrio Mosaic X-5 receiver. The receiver can be connected to any computer through a micro-USB port as shown here. The LED lights near the micro-USB port indicate the receiver is on. To check whether the receiver and connected antenna are receiving any satellites, a web server can be opened on a browser on any Windows computer. The RF input comes through the green wire's micro-SMA port. The Mosaic X-5 GNSS module sits on top of the development kit and extracts information from visible GNSS antennas. The receiver was connected to the RF output from D3A.